A control system that is used to monitor and control a power plant such as an atomic power station (a nuclear power station or a nuclear power plant) is typically duplexed by being constructed as two control devices: one of these is operated as the regular system and the other is operated as a reserve system. In this duplexed control system, the two control devices are connected by an internal-only bus. These two control devices each perform state confirmation of the other, partner control device, by exchanging tracking signals indicating respectively active/inactive, fault information and computation results and/or time point information etc, using the internal-only bus as a tracking bus; synchronization is achieved by updating the calculation results and/or time point information.
In a duplexed control system, when abnormality occurs in the regular-system control device, a message reporting occurrence of abnormality is sent to the reserve-system control device through the internal-only bus, and operation is continued, changing over to employ the reserve system as the regular system. Techniques have been developed whereby, in such system changeover, the regular system/reserve system status is exchanged by the internal-only bus, and, by using a flip-flop circuit of the internal-only bus, either system is set as the regular system while the other is set as the reserve system, so that both systems can be prevented from becoming the regular system at the same time. Such a technique is described for example in Laid-open Japanese Patent Publication Number Tokkai H7-230301 (hereinafter referred to as Patent Reference 1). Also, the technique has been developed of organizing system changeover between the reserve system and regular system in a redundant fashion, by duplicating the system changeover section of the respective control devices of the duplexed control system, so that this is constituted by master and slave changeover sections. Such a technique is disclosed in for example Laid-open Japanese Patent Publication Number Tokkai 2001-356927 (hereinbelow referred to as Patent Reference 2).